Amorphous superlattice structures are known in the art. They are comprised of thin (nanometer range) layers of different semiconductor, metal, or dielectric materials, or mixtures thereof, which exhibit quantum size effects (tunneling, etc.) Prior art superlattice structures have generally been fabricated with silicon-based materials containing hydrogen. These superlattice structures have generally been fabricated with pairs of materials, such as a-Si:H/a-SiN, a-Si:H/a-SiC, and a-Si:H/a-SiO.sub.x. Most of the amorphous superlattice structures of the prior art have used band gap modulated structures as opposed to doping modulated structures in order to observe quantum size effects.
Much less is known about the fabrication of amorphous carbon, or `diamond-like carbon` (DLC), multi-layered superlattice structures. Prior art DLC superlattice structures have a high hydrogen content, the layers of the superstructure being formed from a methane (CH.sub.4)-containing plasma in a plasma enhanced chemical vapor deposition (PECVD) system. The presence of hydrogen in the prior art amorphous hydrogenated carbon (a-C:H) films results in stability problems. In a prior art method of forming a-C:H films, the gaseous plasma has a composition of about 8% methane and 92% argon. The prior art PECVD methods of depositing a-C:H films do not offer the ability to easily and precisely control deposition conditions.
Accordingly, there exists a need for an improved method for fabricating DLC superlattice structures which does not introduce stability problems due to hydrogen content and which offers the ability to easily and precisely control deposition conditions.
Deposition conditions determine the nature of the bonds formed as the gaseous species impinge upon the growing solid. The nature and concentrations of the bonds determines the properties of the individual layers and, subsequently, the properties of the amorphous multi-layered structure. For example, amorphous superlattices using a-C:H multi-layers have an optical band gap which can be varied in the range of 1.2 to 4.0 eV by changing the conditions at which the film is deposited. In a prior art method of forming a-C:H films, the lower electrode, or substrate upon which the superlattice is formed, is driven by a 13.5 MHz power supply while an upper electrode is grounded. A sheath space containing the plasma exists between the lower and upper electrodes. A negative, dc, self-bias voltage is thereby obtained across the sheath space. This RF, self bias voltage can be varied by varying the capacitively coupled power for plasma formation. The deposition conditions are controlled by changing the capacitively coupled power for plasma formation. A negative, dc, self-bias voltage in the range of, for example, -70 to -450 V is obtained across the plasma sheath space by varying this capacitively coupled power. However, precise manipulation of the RF, self-induced bias via the capacitively coupled power for plasma formation is difficult, impractical, and costly. A major defect in this process is the difficulty to accurately control the layer thickness growth to less than one atomic layer during the time required for switching the plasma between the different power conditions. The accurate control of the layer thicknesses as well as the abrupt interface between layers are required to generate the quantum size effects.
Accordingly, there exists a need for an improved method for precisely modulating the conditions during the deposition of the constituent layers of an amorphous multi-layered structure.
Amorphous hydrogenated carbon films are being developed and contemplated for use in masking and passivation overlayers, in gate insulators for metal/insulator/semiconductor (MIS) devices for high temperature applications, in memory applications, and for producing reversible memory devices with electrical or optical information writing and electrical or photoelectrical reading. It has also been recognized that the electrophysical parameters of the a-C:H films can be varied by varying the composition and dimensions of the constituent layers.
Field emission devices (FEDs) are known in the art. Also known is the incorporation of low-work function materials, including DLC, in FEDs to exploit their appreciable electron emissiveness at low voltages. An FED includes a plurality of layers which provide the appropriate electrical conditions for controlled electron emission. These layers have varying electrical properties. Prior art methods of forming, in a field emission device, layers having the required electrical properties include the provision of a variety of chemical species, requiring distinct deposition steps, each of which can require unique deposition conditions and/or equipment.
Accordingly, there exists a need for a method of making a field emission device wherein the constituent layers are fabricated in a continuous manner thereby obviating the need for time-consuming, multiple-step processes.